The present invention relates to mechanical or chemical-mechanical planarization of microelectronic substrate assemblies and, more particularly, to methods for predicting polishing characteristics of polishing pads used in such processes.
Mechanical and chemical-mechanical planarizing processes (collectively xe2x80x9cCMPxe2x80x9d) are used in the manufacturing of electronic devices for forming a flat surface on semiconductor wafers, field emission displays and many other microelectronic substrate assemblies. CMP processes generally remove material from a substrate assembly to create a highly planar surface at a precise elevation in the layers of material on the substrate assembly.
FIG. 1 is a schematic isometric view of a web-format planarizing machine 10 that has a table 11 with a support surface 13. The support surface 13 is generally a rigid panel or plate attached to the table 11 to provide a flat, solid workstation for supporting a portion of a web-format planarizing pad 40 in a planarizing zone xe2x80x9cAxe2x80x9d during planarization. The planarizing machine 10 also has a pad advancing mechanism including a plurality of rollers to guide, position, and hold the web-format pad 40 over the support surface 13. The pad advancing mechanism generally includes a supply roller 20, first and second idler rollers 21a and 21b, first and second guide rollers 22a and 22b, and a take-up roller 23. As explained below, a motor (not shown) drives the take-up roller 23 to advance the pad 40 across the support surface 13 along a travel axis Txe2x80x94T. The motor can also drive the supply roller 20. The first idler roller 21a and the first guide roller 22a press an operative portion of the pad against the support surface 13 to hold the pad 40 stationary during operation.
The planarizing machine 10 also has a carrier assembly 30 to translate a microelectronic substrate assembly 12, such as a thin silicon semiconductor wafer, across the pad 40. In one embodiment, the carrier assembly 30 has a head 32 to pick up, hold and release the substrate assembly 12 at appropriate stages of the planarizing process. The carrier assembly 30 also has a support gantry 34 and a drive assembly 35 that can move along the gantry 34. The drive assembly 35 has an actuator 36, a drive shaft 37 coupled to the actuator 36, and an arm 38 projecting from the drive shaft 37. The arm 38 carries the head 32 via another shaft 39. The actuator 36 orbits the head 32 about an axis Bxe2x80x94B to move the substrate assembly 12 across the pad 40.
The polishing pad 40 may be a non-abrasive polymeric web (e.g., a polyurethane sheet), or it may be a fixed abrasive polishing pad in which abrasive particles are fixedly dispersed in a resin or another type of suspension medium. The polishing pad 40 can have a planarizing surface 42 with a plurality of small raised features projecting from a base portion, or the pad 40 can have a relatively flat planarizing surface 42. FIG. 2A, for example, is an isometric view of a raised feature polishing pad in which the planarizing surface 42 has a plurality of raised features 43 projecting from a base portion of the pad 40. Each raised feature 43 has a small bearing surface 44 to contact the substrate assembly 12. FIG. 2B is an isometric view of a planar polishing pad in which the planarizing surface 42 has a large bearing surface 44 to contact the substrate assembly 12. The planar polishing pad shown in FIG. 2B can also have a plurality of grooves 45 to transport planarizing solution (not shown) under the substrate assembly 12. In either the raised feature pad or the planar pad shown in FIGS. 2A or 2B, abrasive particles may be fixedly attached to the pads such that the bearing surfaces 44 are abrasive.
Referring again to FIG. 1, a planarizing fluid 46 flows from a plurality of nozzles 47 during planarization of the substrate assembly 12. The planarizing fluid 46 may be a conventional CMP slurry with abrasive particles and chemicals that etch and/or oxidize the substrate assembly 12, or the planarizing fluid 46 may be a xe2x80x9ccleanxe2x80x9d non-abrasive planarizing solution without abrasive particles. In most CMP applications, abrasive slurries are used on non-abrasive polishing pads, and clean solutions are used on fixed abrasive polishing pads.
In the operation of the planarizing machine 10, the pad 40 moves across the support surface 13 along the pad travel path Txe2x80x94T either during or between planarizing cycles to change the particular portion of the polishing pad 40 in the planarizing zone A. For example, the supply and take-up rollers 20 and 23 can drive the polishing pad 40 between planarizing cycles such that a point P moves incrementally across the support surface 13 to a number of intermediate locations I1, I2, etc. Alternatively, the rollers 20 and 23 may drive the polishing pad 40 between planarizing cycles such that the point P moves all the way across the support surface 13 to completely remove a used portion of the pad 40 from the planarizing zone A. The rollers may also continuously drive the polishing pad 40 at a slow rate during a planarizing cycle such that the point P moves continuously across the support surface 13. Thus, the polishing pad 40 should be free to move axially over the length of the support surface 13 along the pad travel path Txe2x80x94T.
CMP processes should consistently and accurately produce a uniform, planar surface on substrate assemblies to enable circuit and device patterns to be formed with photolithography techniques. As the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the photo-patterns to within a tolerance of approximately 0.1 xcexcm. Focusing photo-patterns to such small tolerances, however, is difficult when the planarized surfaces of substrate assemblies are not uniformly planar. Thus, to be effective, CMP processes should create highly uniform, planar surfaces on substrate assemblies.
In the highly competitive semiconductor industry, it is also desirable to maximize the throughput of CMP processing by producing a planar surface on a substrate assembly as quickly as possible. The throughput of CMP processing is a function of several factors, one of which is the ability to accurately stop CMP processing at a desired endpoint. In a typical CMP process, the desired endpoint is reached when the surface of the substrate assembly is planar and/or when enough material has been removed from the substrate assembly to form discrete components on the substrate assembly (e.g., shallow trench isolation areas, contacts, damascene lines, etc.). Accurately stopping CMP processing at a desired endpoint is important for maintaining a high throughput because the substrate assembly may need to be re-polished if it is xe2x80x9cunder-planarized.xe2x80x9d Accurately stopping CMP processing at the desired endpoint is also important because too much material can be removed from the substrate assembly, and thus it may be xe2x80x9cover-polished.xe2x80x9d For example, over-polishing can cause xe2x80x9cdishingxe2x80x9d in shallow-trench isolation structures or completely destroy a section of the substrate assembly. Thus, it is highly desirable to stop CMP processing at the desired endpoint.
Raised feature polishing pads, like the one shown in FIG. 2A, are relatively new and have the potential to produce highly planar surfaces because the small spaces between the raised features 43 hold a portion of the planarizing solution on the pad 40 to provide a relatively uniform distribution of planarizing solution under the substrate assembly 12 during planarization. The raised feature polishing pads, however, may have relatively short life cycles and they may produce unpredictable results. For example, the small raised features 43 shown in FIG. 2A generally wear down much faster than the large bearing surface 44 of the planar pad shown in FIG. 2B. The faster wear rate of the raised features 43 reduces the life cycle of raised feature pads. Moreover, any discrepancies of downforce, residence time or other planarizing parameters can produce substantially difference wear levels across a raised feature polishing pad over a number of planarizing cycles. The different wear levels of the raised features will generally result in significantly different polishing rates either across the pad or from one planarizing cycle to another. Such changes in the polishing rate may make it difficult to predict the endpoint of planarizing cycles and/or produce planar surfaces on the finished substrate assemblies. Thus, raised feature polishing pads may produce unpredictable results.
The present invention is directed toward methods for predicting polishing characteristics of polishing pads in mechanical and/or chemical-mechanical planarization processes, and to methods and machines for planarizing semiconductor wafers and other microelectronic substrate assemblies. One aspect of a method in accordance with the invention includes ascertaining a surface parameter of a bearing surface of at least one raised feature projecting from a base portion of a raised feature polishing pad. The raised feature, for example, can be a pyramidal structure having a first cross-sectional area at the base portion of the pad and a second cross-sectional area at the bearing surface. The first cross-sectional area is generally greater than the second cross-sectional area. To ascertain the surface parameter of the bearing surface, an indication of the surface area of the bearing surface may be determined. The surface area of the bearing surface can be estimated by illuminating the bearing surface with a light source and detecting an intensity of the light reflected from the bearing surface. The intensity of the reflected light is generally proportional to the surface area of the bearing surface, and thus the surface area of the bearing surface can be estimated by correlating the detected intensity of the reflected light with a predetermined relationship between the surface area and the light intensity. The actual surface area of selected bearing surfaces can also be measured by viewing the bearing surfaces through a confocal microscope or another type of optical device, or using some other means.
Several polishing characteristics of raised feature polishing pads can be predicted using either an estimated or an actual measurement of the surface area of the bearing surfaces. One aspect of the present invention is the discovery that the surface area of the bearing surfaces is generally proportionate to the polishing rate for the polishing pad. As such, the polishing rate of a polishing pad, or even the polishing rate of a particular region on the polishing pad, can be predicted by measuring the surface area of the bearing surfaces. The estimated polishing rate can then be used to determine whether the pad is suitable for a particular application, or the estimated polishing rate can be used to adjust the time of the planarizing cycle for more accurate endpointing of CMP processing. Therefore, determining the size or surface area of the bearing surfaces is expected to enhance the consistency and predictability of planarizing substrate assemblies using raised feature polishing pads.